1. Field of the Invention
The present invention relates to precise, temperature-dependent voltage references and to voltage comparators and more particularly to a combination circuit useful in temperature and voltage sensitive applications that involve an analog input signal and a relationship to some predetermined threshold condition.
2. Background Art
The combination of a voltage reference circuit and a voltage comparator circuit has been utilized in numerous applications. As a stand-alone circuit, it is used as a voltage threshold detector. The output from such circuitry may be used to generate a logic signal indicating either an over or under reference voltage condition. Such signals are useful in signal processing applications to convert an analog signal to a digital signal based on a precise threshold voltage definition. However, the usual comparator circuitry's high gain difference voltage amplifier can also be used to control a process. In a series voltage regulator, for example, the combination may be used to control the flow of current through a series pass stage coupled between a direct current energy source and the regulated voltage output. In such applications, priority is heavily centered on the generation and utilization of the standard direct current supply voltages (5 volts, 12 volts and 15 volts). The voltage needs of a bipolar integrated circuit over the operating temperature extremes, however, has forced such designs to be nearly temperature independent.
Voltage references have long been a key concept in the development of complex circuits and systems. Ideally, the voltage reference device exhibits a fixed voltage independent of both temperature and current flow. In standard practice, this is easily achieved by placing a temperature compensating forward biased diode in series with a 5.6 volt zener diode. The forward biased diode, for example a lN4148, exhibits a temperature coefficient of approximately minus 2.2 microvolts per degree centrigrade and exhibits a gradual breakdown voltage which is a logarithmic function of the diode current. The 5.6 volt zener diode, for example a lN5232, exhibits a positive temperature coefficient of between plus one and plus two and one-half millivolts per degree centigrade and an abrupt breakdown voltage characteristic. Alternately, a band gap, integrated circuit voltage reference, for example National Semiconductor's LM185H-1.2 can be used. Such implementations exhibit an abrupt breakdown voltage (typically 1.235 volts) and a significant lack of temperature dependence (typically 0.025 millivolts per degree centigrade). This voltage, in turn, may be adjusted to a different value using an operational amplifier such as that manufactured by National Semiconductor under their part no. LM346N. Both of these functions are represented in National's part no. LM10CLN. Unfortunately, the same gain used to multiply the reference voltage also multiplies the temperature coefficient of the voltage reference. The next closest approximation to the idealized voltage reference is represented by National's LM103H-1.8 to LM103H-5.6 series of integrated circuit reference diodes covered by U.S. Pat. No. 3,571,630. These devices approach the idealized behavior of a temperature dependent, voltage reference in the voltage range between 1.8 and 5.6 volts. Each of these devices exhibit abrupt breakdown voltage characteristic with a fixed temperature coefficient, typically of minus 5 millivolts per degree centigrade. The poorest approximation to a voltage reference device in this voltage range is represented by the lN5211-to lN5231 series of zener diodes. Here the current versus voltage characteristics exhibit the gradual breakdown voltage associated with a zener conduction current mechanism. Also, the temperature coefficient is negative, ranging from zero to minus three millivolts per degree centigrade. However, these zener diodes can be used as a temperature dependent voltage reference if the zener diode's current flow is regulated within the required limits. Furthermore with the series addition of a positive temperature coefficient compensating thermister for example, the "tempsistor" made by Midwest Components Incorporated, the temperature dependence of the reference can be made insignificant. The 25.degree. C. resistance values in this series of devices exhibit a positive temperature coefficient resistance in the vicinity of 0.75% per degree C. for the zero to 50.degree. C. range. Tolerances of purchased items on the 25.degree. C. resistance are available from one to 10 percent.
The voltage comparator function is generally performed by an integrated circuit such as National Semiconductor's LM393N. This is a low power, low offset voltage device containing two independent precision voltage comparators. It is designed to operate from a single power supply of between 2 and 36 volts D.C. with a typical supply current of 0.4 milliamperes. The two input voltages to the comparator are limited to the common mode range of between zero volts and the supply voltage less 1.5 volts. The small signal response time is on the order of 1.3 microseconds with a voltage gain in excess of 50 volts per millivolts. Both the supply current and common mode range requirements are objectionable in battery powered, low voltage applications.
The supply current required for the voltage comparator function can be significantly reduced by the selection of an operational amplifier integrated circuit. In this situation, the output must be coupled to the base emitter diode of an NPN transistor due to limitations on the output voltage swing and/or the current sinking abilities of such devices. The NPN transistor's collector acts as the new output and the polarity of the operational amplifier's inputs are then reversed. Recently a device manufactured by Texas Instrument under their part no. TLC251 (a CMOS operational amplifier) operates with a supply current of below thirty microamperes. Unfortunately, the common mode restrictions still limit the maximum input voltage to some voltage less than the supply voltage.
Many of the present designs which combine the voltage reference and voltage comparator functions appear to be limited to the area of low power operation. Thus none of the normal circuitry seems to be able to provide a simple, low cost, low voltage circuit involving only a maximum of three terminals, a specific temperature coefficient (such as minus 7 millivolts per degree centigrade) and a precision threshold voltage adjustment. Accordingly the object of the present invention is to provide a low cost, low power, three terminal circuit which responds as a function of the difference between a given input voltage and a lesser predetermined temperature dependent reference voltage. This response permits the output to act as a difference voltage dependent current sink for threshold voltage detection in power supply applications. Such an arrangement can also be used to implement a voltage reference diode by shorting the input and output terminals together.